I have an expression that I just want it to show blank/nothing if it's negative from a ROUND function. Is this possible?

ROUND(IIF(a<b,"",4.2))

In my Test, if a > b, I get 4 rounded and that's great but if a<b, it shows an error. I understand it's because you can not round blank/nothing but is there a way around this? I can put a value like 0 but I really want it to show nothing.

------------------------------Using VS2010 / Learning in VB.Net / Win2008 R2 / SQL 2008 R2Be kind to the newbies because you were once there.